The present invention relates to a head height adjustment construction in a magnetic recording and reproducing apparatus with a rotary head for use in a video tape recorder (hereinafter referred to as "VTR") or the like.
FIG. 4 of the accompanying drawings illustrates a conventional head height adjustment construction, the view showing in cross section a magnetic recording and reproducing apparatus having a rotary drum, prior to fine adjustment thereof, for use in a VTR. The prior arrangement includes head attachment plates 2 each supporting a head 1 on one end thereof and fastened at an opposite end to the lower surface of an upper rotary drum 4 with an attachment screw 3. The head attachment plates 2 extend parallel to a plane A in which the rotary drum 4 is rotatable. Each head 1 defines a gap having a lower end spaced from a reference surface 5a of a lower fixed drum 5 by a distance which is greater than a prescribed height H by a small interval (about 10 m). By depressing each of the head attachment plates 2 with an adjustment screw 6 threaded in a threaded hole defined in the upper rotary drum 4, the height of the head 1 exposed in a head attachment window 7 can be adjusted little by little in the direction of the arrow B. Such height adjustment is completed when the height of the head 1 has reached the prescribed height H. It would actually be of importance to measure the head height from a lead 8 of the lower fixed drum 5. Since it would be highly difficult to measure such a height, however, the heat height is normally measured with a microscope from the reference surface 5a of the lower fixed drum 5.
A rotatable shaft 10 is fixed centrally to the upper rotary drum 4 by a sleeve 9 and is rotatably supported on the lower fixed drum 5 by bearings 11, 12. A rotary transformer 13 for transmitting signals is disposed between the upper rotary drum 4 and the lower fixed drum 5. A bushing 14 is mounted on an end of the shaft 10 which projects from the lower fixed drum 5. A head cable 15 is connected to the heads 1.
FIG. 5 shows the manner in which a tape travels in the magnetic recording and reproducing apparatus. A magnetic tape 16 travels in the direction of the arrow D while wound around the upper rotary drum 4 and the lower fixed drum 5 through more than 180.degree., with a lower tape edge extending along the lead 8. Since the upper rotary drum 4 rotates at high speed in the direction of the arrow D to rotate the heads 1, the gaps of the heads 1 where thet are slidably contact the tape 16 slide on a magnetic surface of the magnetic tape 1 for recording and reproducing signals on and from the magnetic tape 1.
With the prior head height adjustment construction, the surfaces of the upper rotary drum 4 which confront the head attachment plates 2 lie parallel to the plane A. Therefore, when fine adjustment of the head height is completed by depressing the free end of the head attachment plate 2 toward the lower fixed drum 5 with the adjustment screw 6, a surface 1a of the head 1 for slidable contact with the tape 16 is directed dowardly as shown in FIG. 6. Inasmuch as the lower edge of the magnetic tape 16 is especially subjected to smaller tension and held against the heads 1 under smaller presser in the position E where the magnetic tape 16 starts engaging the drums 4, 5, the head 1 is liable to disengage from the magnetic tape 16 in the vicinity of a lower end n of the gap of the head 1. Designated at m is an upper end of the gap of the head 1. As the clearance between the gap (m - n) of the head 1 and the magnetic tape 16 is increased, the head 1 has difficulty in picking up the magnetic flux from the magnetic tape 16, resuling in a so-called spacing loss which reduces a reproduced output produced from the magnetic tape 16 as it starts engaging the drums 4, 5. As a consequence, the waveform of the reproduced output has rounded portions as shown in FIG. 7 and the head sensitivity is reduced particularly when the magnetic tape begins to engage the drums 4, 5, with the result that a reproduced image will suffer from noise in its upper portion.